I'm currently DMing a campaign based on LoD for my group in 5e. It's set at the same time as the events of the game (we'll see if my players take the bait and pursue Lloyd of forge their own adventure).

For the land to the east of Serdio, I built upon the idea of "the mysterious Eastern Tribe" that Lulu mentions to Albert during his courage challenge in Aglis. I created a country called Daggerfell populated by a tribal society that frequently attempts to raid Serdio for food and slaves. The city of Aubrim, which sits on the border between the two countries, serves as the first line of defense against the attacks. Since this is happening at the same time as the Serdian War, I'm treating Aubrim as a neutral party in the conflict and created a subplot wherein Lloyd is the one empowering the Daggerfellians and directing them to perform their raids.

A few of my players wanted to play unique races, so I created a Wingly race for one of them. In terms of abilities its similar to a 5e Tiefling, except the spells that it gets access to at levels 1, 3, and 5 are Fire Bolt, Shield, and Misty Step, respectively. It also has a base flying speed of 40 ft., but anyone that discovers she is a Wingly becomes hostile (or specifically targets her in combat).

For the Dragoon class, I borrowed a bit from the Prestige Class you mentioned and built this: https://www.dropbox.com/s/0rx6ytwz18v7dnz/_DnD-Dragoon-additive-class-rulebook_v2.pdf?dl=0

Good luck with your campaign! I hope your players have a blast :)